Notices of Memoirs — E. A. N. Arho' — Calm Plants. 553 



principal axis of such a compound crystal shows, when moved 

 across the microscope-stage in convergent polarised light, a gradual 

 transition from a biaxial to a uniaxial figure, the coloured rings 

 at the same time moving outwards and becoming further apart 

 owing to the diminution in the strength of the double refraction, 

 which is positive throughout ; finally, when the rings have all 

 moved out of the field of view, the black cross also disappears, 

 and the corresponding portion of the section is optically isotropic. 

 The mean refractive index has about the same value in all portions 

 of the section. 



Zircon of the first type has been previously described by 

 Professor A. H. Church (1875) and by Dr. S. Stevanovio (1903), 

 and from the researches of these and other authors it would seem, 

 that there are, at least, thl-ee modifications of zircon, viz. : — 



a. Those of specific gravity 4-0, which do not increase in density 

 when ignited. 



yS. Those of specific gravity 4-7, also not increased in density 

 when ignited. 



7. An unstable form of specific gravity about 4'3, which when 

 ignited is increased in density to 4'7. 



That these different kinds are often intergrown in the same crystal 

 is shown by the frequent occurrence of zonal structures in zircon, 

 and further by the behaviour of the crystals when heated. A crystal 

 consisting of an intergrowth of a-zircon and o^-zircon will be increased 

 in density on ignition, but not to the higher limit of 4-7 ; on the 

 other hand, an intergrowth of y3-zircon and 7-zircon will reach the 

 higher limit when ignited. 



In crystalline form and chemical composition (as far as could 

 be determined by qualitative tests) «-zircon and /3-zircon are 

 identical, and these appear to be also the same for 7-zircou. 



III. — On Derived Plant Petrifactions from Devonshire. By 

 E. A. Newell Arber, M.A., F.L.S., F.G.S.i 



SOME interesting plant petrifications in which the structure has 

 been to some extent preserved by means of a mineral agent 

 have recently been discovered in the higher beds of the Upper Culm 

 Measures (Upper Carboniferous) in Western Devon. Although the 

 preservation is not sufiiciently good to render this discovery of any 

 botanical importance, the manner in which the fossils occur is 

 interesting from a geological point of view. The plant remains 

 consist of small rolled fragments of stems, of an inch or less in 

 length, arranged without order in a fine-grained sandstone. They 

 are in all probability derived from some pre-existing beds, and are 

 not contemporaneous with the sandstone in which they are found. 

 Such derived plant remains are very rare, if not unknown, from the 

 Palaeozoic rocks. 



1 Abstract of report read before the British Association, Cambridge, Section C 

 (Geology), August, 1904. 



